The present invention relates, in general, to valve operating mechanisms as applied to an inlet valve upstream from a large fluid-driven turbomachine; and, in particular, the present invention is directed to a valve operator which provides for fast closing and precise control of the inlet valve.
One requirement for operating a large steam turbine is that upstream from the steam turbine in the steam inlet line at least one control valve must be provided having a capability for almost instantaneous shutdown of the steam flow to the turbine under emergency trip conditions. Moreover, the valve must be capable of precise regulation of the steam flow especially under conditions approaching the rated speed of the turbine. It has been found that it may be advantageous to apply butterfly-type valves in a steam inlet line in place of certain prior art rod and plunger-type valves. The advantages of utilizing a butterfly-type valve as an inlet flow control valve have been pointed out in U.S. Patent Application Ser. No. 588,767 to Davis and Mossey filed June 20, 1975 now U.S. Pat. No. 4,020,869 and assigned to the assignee of the present invention. However, because the butterfly valve is disposed directly across the steam flow path certain unique operating characteristics must be considered in the design of a valve operator.
Under normal operating conditions, the steam inlet valve will be fully opened and the pressure drop across the valve will be relatively low. As the valve may be closed in order to throttle the inlet steam flow, the pressure drop across the valve becomes increasingly significant. One resultant condition which occurs as the pressure drop across the valve increases is an increase in bearing friction between the valve shaft and the valve casing.
Because of the foregoing conditions, it has been found that for controlling modern steam valves upstream from a turbine at relatively low rates of movement, it is necessary that a valve operator be capable of applying forces of greater magnitude during slow movement than those forces which are required for emergency tripping. It is undesirable to employ a more massive closing spring to achieve greater operating forces since this would require a larger piston and more heavy-duty hydraulics which might adversely affect the reliability of the system and increase the time required for valve closing. The present invention increases the operating forces necessary for the slow positioning of steam valves by providing an additional hydraulic force in the valve closing direction through the use of a double-acting hydraulic cylinder.
Hence, it is one object of the present invention to provide a valve operator capable of fast closing a steam inlet valve whereas the same valve operator is also capable of precise slow positioning of the steam inlet valve.
Another aspect of the present invention is the evacuation of hydraulic fluid during an emergency trip. A servo valve could be employed to provide for emergency fluid evacuation but because of the large flow necessary to reduce valve closing time, the servo valve would have to be very large. The present invention uses a disk-dump valve of the kind shown in U.S. Pat. No. 3,495,501 to Kure-Jensen issued Feb. 17, 1970. However, unlike the invention shown in U.S. Pat. No. 3,495,501, a separate drain tank must be provided for the hydraulic fluid in the present invention wherein a double-acting hydraulic cylinder is also provided for slow movement control.
It is also an object of the present invention to provide a valve operator having an improved hydraulic fluid evacuation system.
Another aspect of the present invention also concerns an emergency trip wherein it is possible to draw in air to the hydraulic cylinder above the piston during an emergency trip. In order to obviate this possibility an effective fluid seal is applied to the valve operator in combination with the drain tank.
It is a further object of the present invention to provide an effective fluid seal above the double-acting hydraulic cylinder to prevent the ingress of air into the volume above the piston during an emergency trip.
Other objects, advantages and features of the present invention will become apparent from the following description of a preferred embodiment thereof when taken in connection with the accompanying drawing.